A number of types of implantable devices are in use to monitor and control the electrical activity of the heart. For example, it is known to have an implanted pacemaker interconnected via a transvenous pacing lead to an electrode in intimate contact with the myocardial tissue of the heart. The electrode can both sense the electrical activity of the heart and deliver an electrical stimulus provided by the pacemaker when required. Other systems include pacemakers and transvenous pacing leads which have a variety of sensor electrodes proximally spaced behind the tip electrode of the pacing lead. The sensors provide information to the pacemaker.
There are also systems which monitor and provide automatic defibrillation utilizing an implanted power source and an electrode system, either attached to the surface of, or implanted within, the heart. Still other systems combine the pacemaker function with an automatic defibrillation capability, and may include multiple leads extending to internal as well as external portions of the heart.
Combination pacing and defibrillation systems having a combination pacing and sensing lead implanted into the ventricle, and a large surface area patch electrode affixed to or near the exterior surface of the heart, both of which are connected to a pacemaker and/or a defibrillator are also in use. Additionally, pacing systems may include a transvenously implanted lead which provides pacing and sensing only within the atrium. With this type of system, there may be two different pacing or sensor leads extending intravenously into the interior of the heart, in addition to a patch lead and electrode affixed to or near the epicardial surface of the heart, all connected to the pacemaker and/or defibrillator. During the implantation procedure, the attending physician may implant a combination lead having pacing and sensing electrodes, which also includes a defibrillation electrode mounted just proximally of the distal tip in the ventricle, and then test whether the defibrillation electrode can provide sufficient energy to defibrillate the heart to terminate ventricular fibrillation. In the event that defibrillation requires too much energy or cannot be accomplished by the combination lead, a second lead having a patch electrode to be affixed to or near the epicardial surface of the heart or nearby, such as in a subcutaneous or subcostal site, may be required. If such a patch electrode is also required, following affixation of the patch electrode, the attending physician may test various bipolar combinations of the leads for defibrillating the heart, using alternatively the patch electrode and/or the defibrillation electrode on the combination lead as the cathode(s) or anode(s) to determine the lowest threshold for defibrillation.
Thus, while it may be necessary to have the patch electrode affixed to or near the exterior surface of the heart (or subcutaneously or subcostally near the heart), preferably if defibrillation can occur by the use of a combination pacing and defibrillation electrode placed in the right ventricle, the necessity for opening the chest cavity and affixing the patch electrode on or near the heart may be avoided.
When utilizing a ventricular pacing lead which also employs a ventricular defibrillation electrode to accomplish pacing, cardioversion or defibrillation, it is important to recognize that preserving atrial--ventricular synchronization, by proper timing of the respective contractions, is very important to prevent the patient from experiencing adverse effects resulting from non-synchronous ventricular contractions. Thus, in addition to providing the necessary ventricular pacing and defibrillation charges, it is extremely beneficial to have a system which can effectively preserve synchronization of the atrial and ventricular contractions by sensing atrial depolarizations, and timing any necessary electrical pacing stimuli to the ventricle in a manner whereby atrial--ventricular synchronization is maintained.
One method of obtaining the additional sensory information required to provide synchronization has been through the utilization of an atrial sensing lead, to provide sensing within the atrial cavity, which provides additional information to the pacemaker. The atrial sensing lead may be passively or actively implanted within the atrial cavity. However, the disadvantages of having a second intravenously implanted lead include the fact that more hardware is implanted, perhaps to the detriment of cardiac function and optimal blood flow, in addition to the potential problems with its placement or implant location, and may result in increased possibility of infections.
Accordingly, it would be very beneficial to provide a pacing system and cardioversion or defibrillation system which utilizes an improved ventricular pacing and defibrillation lead, having the additional capability of being able to sense atrial electrical activity, thereby assisting the preservation of atrial--ventricular synchronization while eliminating the need for an additional atrial sensing lead.